1. Field of the Invention
This invention relates generally to drive circuits for capacitive loads and more particularly to methods and apparatus for increasing the efficiency of drive circuits for capacitive loads utilizing an inductor and an externally controlled dual branch switching network.
2. Description of the Prior Art
In many circuit applications, it is necessary to drive a load with a square wave signal. When the load exhibits the characteristics of a capacitor, however, the load will acquire and hold the charge supplied on the initial rise of the square wave. It has thus been necessary to discharge the capacitive load so that it may be cyclically driven. The commonly used approach to achieve this result has been to drive the capacitive load with a circuit having a resistive or active pullup and an active pulldown. Such circuits supply current from a voltage to charge the capacitive load and short current to ground to discharge the capacitive load. While such circuits give the desired results, charging and then discharging the capacitive load dissipates considerable power, the magnitude of which can be shown to be greater than or equal to C.sub.1 V.sup.2 f, where C.sub.1 is the capacitance of the capacitive load, V is the peak-to-peak voltage across the capacitive load, and f is the frequency of the square wave driving signal.
The present invention, utilizing a reactive drive concept, greatly decreases or eliminates entirely the energy lost when the capacitive load is charged and discharged. This is accomplished by using an externally commanded switching network means to control the current flow between an energy reservoir and the capacitive load through an inductor. In response to a permissive state of the switching network means, current initially flows through the inductor to charge the capacitive load because the potential of the energy reservoir is above that of the capacitive load. Such current flow causes energy to be stored in the inductor. Once the capacitive load is charged to a potential equal to that of the energy reservoir, current is no longer induced to flow because of a difference in potential. However, the net current continues to flow, being induced by the release of the energy previously stored in the inductor. When the energy in the inductor is fully dissipated, the voltage across the capacitive load will be twice the voltage supplied by the energy reservoir and the current flow ceases.
When the switching network means allows the capacitive load to be discharged, the inductor again stores energy until the voltage across the capacitive load is equal to the voltage of the energy reservoir. The energy stored in the inductor is then utilized to maintain current flow, thereby siphoning the charge from the capacitive load and forcing it back into the energy reservoir where it is available to charge the capacitive load on subsequent cycles. Integral to this circuit is the externally controlled switching network means whereby the resonant characteristics of the LC combination may be controlled by allowing variations in characteristics of the square wave generated. Separate means are provided to restore the inherent energy losses of the circuit.
In a different context, this principle was utilized by Z. D. Farkas in his "Voltage Multiplying Inverter/Converter System," (U.S. Pat. No. 3,377,541). However, the various embodiments of the present invention make use of either one or two independently controlled single pole-single throw switches rather than a double pole-double throw switch as claimed by Farkas. Furthermore, the essence of the Farkas circuit is to utilize the energy stored by an inductor in a resonant LC circuit to boost the output voltage across a capacitor until the inherent losses in the circuit equal the energy supplied by the source. Consequently, energy is never returned to the source of any other energy reservoir. In fact, the placement of diode 41 (FIG. 4 of the Farkas invention) will prevent energy from ever returning to the only possible energy reservoir in the Farkas circuit, namely the power source.
Accordingly, it is the general object of the present invention to increase the efficiency of capacitive load driver circuits by retrievably storing energy which would otherwise be lost.
It is yet another object of the present invention to utilize the resonant characteristics of an LC circuit to force the capacitive load to discharge into an energy reservoir where the discharged energy can be used to recharge the capacitive load in subsequent cycles.
It is yet another object of the present invention to provide a means whereby the rise and fall times of a capacitive load driven by a square wave may be precisely controlled.
It is still another objective of the present invention to provide a driver circuit utilizable with all of the large variety of waveforms which may be used to clock charged coupled devices.